The present invention relates to a planar lighting device including light sources and a light guide plate for admitting light emitted by the light sources and emitting a light through the light exit plane thereof. The inventive planar lighting device is used for indoor and outdoor illumination or as a backlight to illuminate the liquid crystal display panel used in liquid crystal display devices or a backlight used for advertising panels, advertising towers, advertising signs, and the like.
Liquid crystal display devices use a backlight unit for radiating light from behind the liquid crystal display panel to illuminate the liquid crystal display panel. A backlight unit is configured using optical members including a light guide plate for admitting light emitted by an illuminating light source and diffusing the light within the inside thereof to emit the light through a light exit plane, irradiating the liquid crystal display panel, a prism sheet, and a diffusion sheet.
Currently, large liquid crystal display televisions predominantly use a so-called direct illumination type backlight unit having no light guide plate but including optical members such as a diffusion plate disposed immediately above the illuminating light source. This type of planar lighting device includes cold cathode tubes serving as a light source provided on the rear side of the liquid crystal display panel whereas the inside of the backlight unit provides white reflection surfaces to secure uniform light amount distribution and a necessary brightness.
To achieve uniform light amount distribution with the direct illumination type backlight unit, however, the backlight unit needs to have a given thickness, say about 30 mm, in a direction perpendicular to the liquid crystal display panel. While demands of still thinner backlight units are expected to grow in the future, achieving a further reduced thickness of say 10 mm or less with a backlight unit is deemed difficult in view of uneven light amount distribution expected to accompany the direct illumination type.
Among backlight units that allow reduction of thickness thereof is a backlight unit using a light guide plate for admitting light emitted by light sources from its side face to diffuse it, and emitting the light through a light exit plane positioned on a front side different from the plane through which the light has been admitted.
There has been proposed a backlight of a type described above using a light guide plate formed of a material prepared by mixing scattering particles for scattering light into a transparent resin, for which reference may be had, for example, to JP 07-36037 A, JP 08-248233 A, JP 08-271739 A, and JP 11-153963 A.
JP 07-36037 A, discloses a light diffusion light guide light source device including a light diffusion light guide member having at least one light entrance plane region, and at least one light exit plane region and light source means for admitting light through the light entrance plane region, the light diffusion light guide member having a region that has a tendency to decrease in thickness with the increasing distance from the light entrance plane.
JP 08-248233 A discloses a planar light source device including a light diffusion light guide member, light source means for supplying light through at least one side of the light diffusion light guide member, a prism sheet provided on the side of the light diffusion light guide member closer to a light exit plane, and a reflector provided on the rear side of the light diffusion light guide member. JP 08-271739 A discloses a liquid crystal display including a light emission direction correcting element formed of sheet optical materials provided with a light entrance plane having a repeated undulate pattern of prism arrays and a light exit plane given light diffusing properties. JP 11-153963 A discloses a light source device including a light diffusion light guide member having a scattering power therein and light supply means for supplying light through an end plane of the light diffusion light guide member.
In the planar lighting devices provided with a light diffusion light guide plate containing light scatterers mixed therein as disclosed in the above prior art literature, light emitted by the light source and admitted through the light entrance plane into the light diffusion light guide member receives scattering effects of the light scattering particles to be scattered as the light propagates through the inside of the light diffusion light guide member. However, only a portion of the scattered light having a predetermined angle or more to the light exit plane of the diffusion light guide member is emitted from the light exit plane, while portions of the scattered light having a predetermined angle or less and the light directed to the reflector of the rear side are fully reflected, and a significant proportion of the light that has reached is returned back into the diffusion light guide member.
The above composite process produces light that propagates with a directivity to travel obliquely forward as viewed from the light source, and is highly efficiently emitted as almost uniformly bright light through the full light exit plane. Briefly, light radiated by the light source can be extracted as almost uniformly bright light through the light exit plane of the light diffusion light guide member.
Thus, the prior art literature described above purportedly states that a light guide plate containing light scattering particles mixed therein is capable of emitting uniform light with a high light emission efficiency.
As regards the light guide plate used in the planar lighting device, there have been disclosed a light guide plate in the form of a flat plate, and a light guide plate composed of a portion shaped to have a region with a tendency to grow thinner with the increasing distance from the light entrance plane attached to the other portion, in addition to the light guide plate described above that is shaped to have a region with a tendency to grow thinner with the increasing distance from the light entrance plane.
However, in the planar lighting device using any of the light guide plates disclosed in the above prior art literature, the light guide plate is expanded/contracted or warped due to effects of temperature and humidity. The expansion/contraction or the warp of the light guide plate occurs to a greater extent as its size increases. Thus, the effect of expansion/contraction of the light guide plate becomes conspicuous when the planar lighting device using the light guide plate has greater dimensions.
Thus, when the light guide plate is expanded/contracted, if the light guide plate and the light source such as a light emitting diode (LED) disposed adjacent to its light entrance plane are fixed to different support members such as housings, during expansion of the light guide plate, the expanded light guide plate presses the adjacent light source such as an LED to destroy it.
When the light guide plate is contracted, a distance between the light entrance plane of the light guide plate and the light source such as an LED is increased to lower efficiency of admitting the light emitted from the light source into the light entrance plane of the light guide plate, thus causing a problem of a reduction in use efficiency of the light emitted from the light source.
When the light guide plate is warped, a distance from the liquid crystal display panel illuminated by the planar lighting device is partially changed to disable uniform illumination, resulting in uneven brightness. Moreover, when the warp is enlarged, the light guide plate comes into contact with the liquid crystal display panel to destroy it.
In the planar lighting device disclosed in the prior art literature, no consideration is given to the problems caused by the expansion/contraction or the warp of the light guide plate. Thus, it is impossible to solve the problems.